This invention is in the field of printing, and is more specifically directed to high-speed and high-precision electrographic printing.
Electrographic printing has become a prevalent technology in the modern computer-driven printing of text and images, on a wide variety of hard copy media. Tis technology is also referred to as electrographic marking, electrostatographic printing or marking, and electrophotographic printing or marking. Conventional electrographic printers are well suited for high resolution and high speed printing, with resolutions of 600 dpi (dots per inch) and higher becoming available even at modest prices. At these resolutions, modern electrographic printers and copiers are well-suited to be digitally controlled and driven, and are thus highly compatible with computer graphics and imaging. Examples of conventional printing machines with this capability include the DIGIMASTER 9110 network imaging system and the DIGIMASTER 9150i digital press, both available from Heidelberg USA, Inc.
A typical electrographic printer includes a primary image forming photoconductor, which may be a moving belt in large scale printers, or a rotating drum in smaller laser printers and photocopiers. The photoconductor is initially sensitized or conditioned by the application of a uniform electrostatic charge at a primary charging station in the printer. An exposure station forms an image on the sensitized photoconductor by selectively exposing it with light according to the image or text to be printed. The exposure station may be implemented as a laser, an array of light emitting diodes (LEDs), or a spatial light modulator. In modern electrographic printing, a computer typically drives the exposure station in a raster scan manner according to a bit map of the image to be printed. The exposing light discharges selected pixel locations of the photoconductor, so that the pattern of localized voltages across the photoconductor corresponds to the image to be printed.
At a developing or toning station in the typical electrographic printer, a developer roller or brush is biased to a bias voltage roughly at the primary charging voltage of the sensitized photoconductor prior to exposure. The biased developer roller or brush is loaded with toner, which is typically a mixture of a fine metallic powder with polyester resin and powdered dye, charged to the bias voltage. As the exposed photoconductor passes the developing station, toner is attracted to the discharged pixel locations of the photoconductor. As a result, a pattern of toner corresponding to the image to be printed appears on the photoconductor. This pattern of toner is then transferred to the medium (e.g., paper) at a transfer station. The transfer station charges the medium to an opposing voltage, so that the toner on the photoconductor is attracted to the medium as it is placed in proximity to the photoconductor.
The transferred toner becomes permanently fixed to the medium at the a fusing, or fixing, station that is located downstream from the transfer station. Conventional fusing stations apply heat and pressure to fuse the transferred toner to the medium, after which the then-printed medium travels to a finishing station in the printer for collating, sorting, stapling or other binding, and other finishing operations.
As mentioned above, modern electrographic printers have extremely high resolution capability. The high resolution that is achievable in the formation of the image on a medium has translated into stringent mechanical requirements on the remainder of the printing machine, including stringent requirements in the precision with which the medium is positioned relative to the photoconductor at the transfer station.
The overall precision of the image formation and mechanical positioning in modern printing machines is of special concern in certain printing applications. One example of a printing application with extreme position is the printing of election ballots that are to be machine read after marking by the voter. Anti-counterfeiting measures implemented by ballot reading machines rely upon extremely precise positioning of printed identifiers on the ballot; for example, modern ballot reading machines typically discard, as counterfeit, those ballots having their identifiers mispositioned by as little as twenty thousandths of an inch. Another printing application that requires extremely high precision is the printing of high-quality images on pre-printed media, in which the printed images may be required to be very precisely positioned relative to the existing images and text.
One important parameter in the positioning of media in a printing machine is referred to as “skew”. Skew refers to error in the rotational positioning, or angular deviation, of a sheet of the media to be printed at the transfer station. FIG. 1 illustrates the effect of skew in a conventional printing machine, for example in the positioning of sheet 2 at a transfer station in the printing machine. Sheet 2 is a sheet of paper or another medium, to which an image is to be transferred from a developed region of a photoconductor. In the illustration of FIG. 1, sheet 2 is skewed, or rotationally misaligned, such that the center line CL of sheet 2 along its major vertical axis has an angular deviation relative to the path center line PCTR. Path center line PCTR refers to the center line of a transport path of media in the printing machine, and thus to the center line of the imaged portion of the photoconductor. In the printing art, measurement of skew is defined by the horizontal distance between the horizontal position of center line CL at the top of sheet 2 to the horizontal position of center line CL at the bottom of sheet 2. This measured skew is shown as distance SKEW in FIG. 1.
In conventional printing machines, skew is controlled by way of one or more motors at a registration station that feeds sheets of media to the transfer station. U.S. Pat. No. 5,322,273, incorporated herein by this reference, describes an example of a conventional registration mechanism for placing sheets of a medium to be printed in registration with a photoconductor in a conventional printing machine. According to this conventional approach, a pair of motors are laterally separated from one another, and advance each sheet along the path toward the transfer station and photoconductor. These motors are individually and differentially controlled in response to the sensed position of the sheet along the path to remove skew in the medium relative to the path.
FIG. 2 schematically illustrates the operation of conventional registration stations, such as that described in U.S. Pat. No. 5,322,273. Sheet 2 is illustrated as being advanced along the path (having path center line PCTR) by rollers 6a, 6b. Rollers 6a, 6b are advancing rollers driven by individual motors, and controlled in the manner described in U.S. Pat. No. 5,322,373. Laterally separated sensors 5a, 5b sense the leading edge of sheet 2 as it is advanced along the path. In the example of FIG. 2, sensor 5b will detect a leading edge of sheet 2 earlier in time than will sensor 5a. A control circuit (not shown) receives signals from sensors 5a, 5b, and controls the motors driving rollers 6a, 6b accordingly to remove the skew. In this example, the conventional printing machine described in this U.S. Pat. No. 5,322,273 eliminates this skew by deactivating the motor driving roller 6b (for example, upon sensor 5b detecting the leading edge of sheet 2) to hold sheet 2 in position at the point of contact of roller 6b, and continuing to drive roller 6a to rotate sheet 2 for an additional time (for example, until sensor 5a detects the leading edge of sheet 2). The driving of roller 6a with roller 6b stopped will effect a rotation of sheet 2, in the clockwise direction in the example of FIG. 2, correcting the angular deviation of sheet 2 relative to path center line PCTR.
By way of further background, another approach to the elimination of skew in printing machines involves the differential driving of laterally spaced rollers to different velocities, responsive to a skew measurement. U.S. Pat. No. 5,078,384 and U.S. Pat. No. 5,094,442, both incorporated by reference herein, describe this differential velocity approach. According to this conventional control method, referring to FIG. 2, the control circuit computes the extent of skew of sheet 2 from signals that it receives from sensors 5a, 5b, and controls the motors driving rollers 6a, 6b such that roller 6a rotates faster than roller 6b, in the skew example of FIG. 2. This differential velocity will effectively rotate sheet 2 to eliminate the skew by the time that sheet 2 exits the registration station.
While these conventional printing machines are effective to remove skew to a significant degree, it has been observed in connection with this invention that the ultimate precision with which the skew is eliminated is still limited, especially relative to the extremely high precision required for some modern printing jobs, as mentioned above. Furthermore, while the precision of skew compensation in these conventional printing machines can be adjusted, such adjustment requires a service technician to take down the machine and effect the specific adjustment. Especially in the context of a print shop environment, this adjustment typically necessitates a service call, thus involving significant cost, as well as machine downtime (at least for precision printing jobs) while awaiting the service technician.